Question

An analytical chemist decides to analyze a given sample spectrophotometrically for its copper content.  The chemist knows that copper(I) ions form a colored coordination complex with neocuproine (ncup), C₁₄H₁₂N₂, in a one-to-two ratio, Cu(ncup)₂⁺.  The chemist also knows that Cu(ncup)₂⁺ is more soluble in 3-methyl-1-butanol than in water and that Cu(ncup)₂⁺ in 3-methyl-1-butanol shows an absorbance maximum at 454 nm.  Thus, the chemist proceeds with the chemical analysis.  She treats 0.1482 g of copper wire, 98.50% (w/w) pure, with a minimum volume of concentrated nitric acid in order to get all of the copper metal into solution (aqueous) as copper (II) ions.  After neutralizing the copper-containing solution with base, the chemist transfers the solution quantitatively to a 500.0-mL volumetric flask and then dilutes the solution with deionized water up to the mark on the flask.  Next, the chemist treats 10.00 mL of the solution with a reducing agent in order to convert copper(II) ions to copper(I) ions and then adds a buffer in order to change the pH of the solution to one for which complexation between copper(I) ions and neocuproine occurs readily.  Subsequently, the chemist treats the resulting solution with neocuproine to complex all of the copper(I) ions and then adds 100.00 mL of 3-methyl-1-butanol.  After shaking well to make sure that essentially all of the Cu(ncup)₂⁺ is extracted by the 3-methyl-1-butanol from the aqueous solution, the chemist separates the aqueous layer (bottom) completely from the 3-methyl-1-butanol layer (top) and then quantitatively transfers the 3-methyl-1-butanol layer to a 1.000-L volumetric flask, diluting the solution with 3-methyl-1-butanol up to the mark on the flask.  Using a spectrophotometer, the chemist finds the percent transmittance of the final 3-methyl-1-butanol solution in a 2.00-cm cuvet is 18.8 at 454 nm.  The chemist then prepares the sample.  She treats 0.2990 g of the previously dried copper oxide sample with a minimum volume of concentrated sulfuric acid in order to get all of the copper into solution (aqueous) as copper(II) ions.  After neutralizing the copper-containing solution with base, the chemist transfers the solution quantitatively to a 500.0-mL volumetric flask and then dilutes the solution with deionized water up to the mark on the flask.  Next, the chemist treats 10.00 mL of the solution with the reducing agent and then adds the buffer.  Subsequently, the chemist treats the resulting solution with neocuproine and then adds 100.00 mL of 3-methyl-1-butanol.  After shaking well to make sure that essentially all of the Cu(ncup)₂⁺ is extracted by the 3-methyl-1-butanol from the aqueous solution, the chemist separates the aqueous layer (bottom) completely from the 3-methyl-1-butanol layer (top) and then quantitatively transfers the 3-methyl-1-butanol layer to a 500.0-mL volumetric flask, diluting the solution with 3-methyl-1-butanol up to the mark on the flask.  Using a spectrophotometer, the chemist finds the percent transmittance of the final 3-methyl-1-butanol solution in a 2.00-cm cuvet is 27.6 at 454 nm.  Find the percent by mass of copper in the sample analyzed by the analytical chemist.

Answer 1

Copper Wire :

0.1482g of 98.5% pure copper wire is taken for the experiment. This means 98.5% of 0.1482g of copper wire is pure copper.

= (98.5/100) * 0.1482 = 0.14597g or 0.146g

0.146g is the amount of copper present in the wire taken for analysis.

Now, 0.146g is dissolved in concentrated nitric acid and further diluted to 500mL using deionized water in a volumetric flask. Amount of copper present in 1 mL of this solution is -

(0.146*1) / 500 = 0.000292g = 0.292mg/mL

If 10mL of this solution is taken for complexation, amount of copper taken is (0.292*10) = 2.92mg

This 2.92mg of copper is treated with neocuproine. As given in the description, a 1:2 complex is formed. This complex is extracted with 100mL of 3-methyl-1-butanol and further diluted to 1000mL in a volumetric flask with the same solvent. Hence, 2.92mg of copper is now present in a complexed state in 1000mL solution. Amount of copper present in 1mL of solution is -

(2.92*1) / 1000 = 0.00292mg/mL

2mL of this solution is taken in a cuvette to measure the absorbance in a spectrophotometer. Amount of copper in 2mL of this solution is = 2*0.00292 = 0.00584mg.

Therefore, in the solution giving an absorbance of 18.8, there is 0.00584mg copper present.

Sample :

Amount of copper present in the dried copper oxide sample should be determined.

The final absorbance of the complex obtained using 2mL of the solution is 27.6.

Using the result obtained above, if there is 0.00584mg of copper in a solution giving absorbance of 18.8, then the solution giving absorbance of 27.6 has -

(0.00584*27.6) / 18.8 = 0.008574mg.

Hence, amount of copper present in 2mL of the complexed solution is 0.008574mg.

This solution is taken from 1000mL of the solution. So, the amount of copper present in 1000mL solution is -

(0.008574*1000) / 2 = 4.287mg

The 1000mL solution was prepared by forming complex with 10mL of original solution of copper. Therefore, 4.287mg of copper is present in the 10mL of the original solution.

But 500mL of the original solution was made using the copper oxide sample. Hence, amount of copper in that 500mL solution is -

(4.287*500) / 10 = 214.35mg = 0.2143g.

So, there is 0.2143g of copper in 0.2990g of copper oxide sample taken for analysis.

Percentage (by mass) of copper in copper oxide is = (0.2143/0.2990) * 100 = 71.67%

Therefore, the copper oxide sample contains 71.67% (by mass) copper in it.

(Note : These calculations can also done, by using the amount of complex formed by copper and further relating the amount of complex to the absorbance value. It will still give the same result.)